The present invention relates to a method and/or architecture for voltage controlled crystal oscillators generally and, more particularly, to controlling voltage controlled crystal oscillators by adjusting an inverting delay.
Voltage Controlled Crystal Oscillators (VCXO) are an important circuit in Set-Top Boxes (STB) to track digital television broadcaster""s master frequency from program to program. In an STB, the VCXO functions as an executor within a Program Clock Recovering (PCR) Loop to trim a local system clock frequency up to +/xe2x88x92100 ppm.
The VCXO has many kinds of physical formats each with different costs. A metal-can VCXO has a best performance but a cost that is not acceptable for a consumer product like an STB. Lower cost clock synthesizers, such as an MK3727 available from Integrated Circuit Systems, San Jose, Calif., that work with a pullable crystal are available at a more reasonable price. Conventional discrete VCXOs may also be available, but both bill of materials (BOM) and total costs are commonly undesirable. A conventional discrete VCXO format having a good performance and modest BOM was disclosed in U.S. Pat. No. 6,181,217 by White. However, a lower cost VCXO is still desired to reduce a cost of the STB.
The present invention concerns a circuit generally comprising a tank circuit and an inverter circuit. The tank circuit may be configured to generate a first signal having a frequency of oscillation in response to a second signal. The inverter circuit may be configured to (i) generate the second signal in response to inverting the first signal and (ii) adjust a delay in generating the second signal in response to an input signal to change the frequency of oscillation.
The objects, features and advantages of the present invention include providing a method and/or architecture for voltage controlled crystal oscillation that may provide (i) a simple design structure, (ii) easy integration into an integrated circuit, (iii) a small silicon area consumption, (iv) a low production cost, (v) a high reliability, (vi) easy manufacturing, (vii) compatibility with digital fabrication processes, (viii) precision control by an analog input, (ix) a low jitter output, (x) good performance and/or (xi) a high performance to cost ratio.